Provenance of Baker River sediments (Chile, 48°S): Implications for the identification of flood deposits in fjord sediments

Abstract

AbstractFloods are among the most destructive natural hazards on Earth. In paleohydrology, sediments are generally considered as one of the best archives to extend flood records to pre‐historical timescales. Doing so requires being able to identify flood deposits from sediment archives and decipher between flood types. The latter is particularly important in glacierized regions, where meteorological floods frequently co‐occur with glacial lake outburst floods (GLOFs). In Patagonia, results from a recent study suggest that GLOFs are recorded in downstream fjord sediments as fine‐grained and organic‐poor layers, representing the high amount of glacier rock flour transported during lake outbursts, whereas meteorological floods are represented by coarser and more organic deposits. However, not all fine‐grained organic‐poor deposits could be associated with historical GLOFs. Here, we reconstruct the provenance of these Baker River flood deposits using 87Sr/86Sr and εNd, taking advantage of the clear lithological differences that exist between both sides of the watershed. Our results show that both 87Sr/86Sr and εNd are suited to reconstruct sediment provenance in the Baker River watershed but that εNd is the most effective and the least affected by grain‐size variations. Our provenance results confirm that the 21st‐century fine‐grained and organic‐poor deposits represent GLOFs and that the largest winter meteorological flood on record has a distinct coarse and organic‐rich signature. However, our results show that rain‐on‐snow events that occur in summer, and therefore primarily affect the western glacierized part of the watershed, have the same fine‐grained organic‐poor signature as GLOFs. Therefore, this study shows that the sedimentary signature of rain‐on‐snow floods in partially glacierized watersheds depends on the season during which they occur. We anticipate that our findings will contribute to a better interpretation of flood records from partially glacierized watersheds.